The reasons for the deactivation of the hydroxynitrile lyase (Hnl) from the rubber tree (Hevea brasiliensis) at low pH values (below 4.0), and the influence of buffer salts as well as the possible stabilization of-the enzyme by additives were investigated. For the elucidation of responsible phenomena, dynamic light scattering and fluorescence spectroscopy were employed. rt was found that the Hnl tryptophan fluorescence maximum shifted to a higher wavelength and the anisotropy increased when the pH value was decreased. Furthermore, the aggregate size of the Hnl particles increased with increasing pH; therefore, we suggest art unfolding of the enzyme followed by aggregation leading to closely packed enzyme particles at low pH values. The large particle sizes at high pH are accounted for by the weak interaction of the enzyme molecules which were found not to influence the enzyme activity.